SCIENCE WATCH

SCIENCE WATCH; Neptune Rendezvous

Published: August 18, 1987

LAUNCHED 10 years ago this Thursday, the Voyager 2 spacecraft has visited the three largest planets, discovered new moons and planetary rings and observed the first active volcano beyond Earth. And still it cruises on, more than two billion miles from Earth, heading for a rendezvous in two years with the planet Neptune.

Flight controllers at the Jet Propulsion Laboratory in Pasadena, Calif., reported yesterday that the unmanned spacecraft was ''working great,'' traveling 41,600 miles an hour and on course for Neptune, now 642 million miles away. A midcourse maneuver last March put Voyager 2 on a trajectory that should take it within 3,000 miles of Neptune on Aug. 24, 1989. The craft will also make close-up observations of Neptune's giant moon, Triton.

After the encounter with Neptune, the first by a spacecraft, Voyager 2 will continue out in search of the heliopause, the outer boundary of the Sun's sphere of influence.

Voyager 2 was launched Aug. 20, 1977 from Cape Canaveral, Fla. Voyager 1, was launched two weeks later, but it traveled a more direct trajectory to arrive at Jupiter and Saturn ahead of its sister craft. The two Voyagers discovered active volcanoes on Jupiter's moon Io, detected a faint ring system encircling the planet and found three new moons. At Saturn, they discovered six new moons and transmitted pictures showing the celebrated rings in dazzling detail.

After Voyager 1 flew by Saturn in November 1980, the planet's gravitational forces sent the spacecraft on a course up and away from the plane in which the planets orbit the Sun. But Voyager 2, which passed Jupiter in July 1979 and Saturn in August 1981, continued on to fly by Uranus Jan. 24, 1986. It discovered 10 new moons around Uranus and provided the first detailed look at the planet's rings.

All told, the Jet Propulsion Laboratory said, Voyager 2 has traveled 3.7 billion miles so far in its journey to the outer reaches of the solar system.